The goal of the proposed work is to develop new methods for precisely altering the genome of zebrafish efficiently. The technology to be developed here will dramatically expand and likely revolutionize the kinds of experiments that can be performed and the kinds of questions that can be asked with the zebrafish. Our approach uses homologous recombination to replace specific sequences in the host genome with introduced exogenous sequences. The immediate aims are: 1) to demonstrate targeted genes can be altered by homologous recombination in somatic tissues, 2) to optimize conditions to accomplish homologous recombination utilizing single strand or double strand donor DNA molecules, 3) to measure the occurrence of genome integration of donor DNA sequences at unintended loci by non-homologous recombination mechanisms, and 4) to demonstrate that heritable alterations can be produced and recovered in subsequent generations. New technologies have recently been developed that permit the very efficient induction of double strand DNA breaks at specific loci using engineered nucleases. The chromosomal DNA breaks stimulate recombination at the site of the lesion and have been used to initiate gene targeting in fruit flies and cultured mammalian cells. Our preliminary results indicate that simultaneous introduction of target-specific nucleases and single strand donor DNA molecules promotes homologous recombination in the somatic tissues of zebrafish embryos resulting in designed sequence changes at a targeted host gene. We anticipate the proposed experiments will identify conditions needed for the routine manipulation of the zebrafish genome.